Many modern day machines, from data printers to positioning devices to robots and the like, require a mechanism for converting rotary motion to linear motion at relatively high speeds and with considerable accuracy. Today's standards require such equipment to have good positional repeatability, reliability and long life. A typical device for accomplishing these objectives is an anti-backlash nut threaded onto a lead screw that traverses back and forth i.e. undergoes bidirectional translation as the screw is rotated in one direction or the other.
One such mechanism is disclosed in U.S. Pat. No. 4,131,031 (incorporated herein in its entirety by reference) which includes an anti-backlash nut assembly which translates along a lead screw in response to the rotation of the screw. The nut itself is divided into two nuts or halves, both of which have internal threads complementary to the thread on the screw. The two portions or halves of the nut are secured in a radial direction on the screw, by a spacer in the form of a tube or collar. A compression spring continuously applies an axial biasing force in the longitudinal direction of the screw, forcing the nut halves apart, so that the nut threads bear against the threads in the screw in opposite directions to minimize or eliminate backlash.
While anti-backlash nuts made in accordance with the above-identified patent have proven satisfactory, they are somewhat limited because of the spring employed. The spring biases the nut halves apart to urge thread flanges against opposite thread flanks of the lead screw. If a load is imparted to one of the nut halves without the spring in place, the opposite nut half translates with respect to the first nut half because there is nothing to restrain it. The anti-backlash feature of this prior art invention is dependent upon the axial force of the spring itself. Since the load is transferred through the spring, a spring of high compressibility or stiffness causes a large frictional force and, thus, loss of efficiency. Conversely, too weak a spring does not adequately reduce backlash. Thus, the anti-backlash aspect of the '031 patent is only as good as the force of the spring.
Another such mechanism is described in U.S. Pat. No. 4,353,264 (incorporated herein in its entirety by reference) in which an anti-backlash nut assembly also undergoes translational movement longitudinally along a screw in response to rotational movement of the screw. The mechanism of the '264 patent employs a nut which is split into two nut halves which are internally threaded with the same thread and hand as the screw. The two nut halves are retained in the radial direction by a coaxial cylinder surrounding portions of the nut halves. A spacer is mounted on the retainer between the surfaces of the nut halves and is rotationally biased by a torsion spring against at least one of the surfaces to close any gap which may occur by wear over time between the thread of the screw and the thread of the nut. In another embodiment a slightly elastomeric washer is provided between the spacer and a nut halve to maintain torque consistency over shaft length. Thus, backlash is minimized by employing a number of mechanical elements.
An anti-backlash mechanism is also disclosed in U.S. Pat. No. 5,732,596 (incorporated herein in its entirety by reference). In this invention two nut halves are also joined together by a helical torsion spring which induces relative rotation of the nut halves in opposite directions toward a closed abutting position. Also, a high friction elastomeric washer is inserted between opposite faces of two nut halves to limit the ability of the nut halves to rotate relative to one another; i.e. in opposite directions and thus separate from one another when an axial force is applied.
Despite the commercial success of the inventions embodied in the above referenced patents, a need exists for a smaller, more compact, easy to assemble, anti-backlash mechanism with fewer parts and which also occupies less space. This need is due in part to the ever increasing movement toward micro-miniaturization of instruments and the attendant need to reduce the load induced on the miniature motors which rotate the drive screws in delicate but precise miniature instruments.